ivar 0.4.0

Ruby instance variables are so convenient - you don't even need to declare them! But... they are also dangerous, because a mispelled variable name results in `nil` instead of an error. Why not have the best of both worlds? Ivar lets you use plain-old instance variables, and automatically checks for typos. Ivar waits until an instance is created to do the checking, then uses Prism to look for variables that don't match what was set in initialization. So it's a little bit dynamic, a little bit static. It doesn't encumber your instance variable reads and writes with any extra checking. And with the `:warn_once` policy, it won't overwhelm you with output.